Deuterated analogues of methoxyflurane useful as an anesthetic

ABSTRACT

Deuterated analogues of methoxyflurane and a method of their use as an anesthetic.

BACKGROUND OF THE INVENTION

Methoxyflurane, 2,2-dichloro-1,1-difluoro-1-methoxy-ethane, is acommonly used general inhalation anesthetic. U.S Pat. No. 3,104,202. Amajor drawback of this compound is the renal dysfunction which mayresult from its use. J. Am. Med. Assn. 216, 278-288 (1971). Studies haveshown a dose related nephrotoxicity in rats anesthetized withmethoxyflurane. Anesthesiology 36, 571-587 (1972). Such toxicity isrelated to the presence of inorganic fluoride released into the bloodserum by the metabolic degradation of the methoxyflurane. Anesthesiology35, 286-292 (1971) and Anesthesiology 33, 579-593 (1970).

It is known that the replacement of hydrogen by deuterium in someantimicrobial compounds will result in a marked decrease in enzymaticdefluorination. See 15th Interscience Conf. of Antimicrobials, Agentsand Chemotherap., September 24-26, 1975, Abstract Nos. 100, 101, and102.

SUMMARY OF THE INVENTION

It has been discovered that certain deuterated analogues ofmethoxyflurane decrease the metabolism of the compound to inorganicfluorides when the deuterated analogue is inhaled by an animal,especially a mammal. The placement of the deuterium atoms in themolecule was found to be critical and apparently unexpectedly is speciesspecific, since one deuterated methoxyflurane analogue was found uponinhalation by an animal to actually undergo an increase in enzymaticdefluorination thereof raising the concentration of inorganic fluoridesin the serum of the animal.

Two deuterated analogues were found to decrease the amount ofmetabolized inorganic fluoride from that realized only frommethoxyflurane in the serum as measured by total fluoride present in theurine following anesthesia. The structures of these compounds are asfollows: ##STR1## hereafter MF-D₄ or2,2-dichloro-1,1-difluoro-1-methoxy-d₃ -ethane-d, and ##STR2## hereafterMF-D₃ or 2,2-dichloro-1,1-difluoro-1-methoxy-d₃ -ethane.

A third analogue having the structure ##STR3## hereafter MF-D or2,2-dichloro-1,1-difluoro-1-methoxy-ethane-d was found to increase totalinorganic fluoride output in the urine.

The compound 2,2-dichloro-1,1-difluoro-1-methoxy-d₃ -ethane, i.e.,MF-D₃, is particularly preferred, and when inhaled by an animal showedthe lowest concentration of inorganic fluoride in the urine.

As used herein, the term "animal" refers to an inhalation anestheticsusceptible animal.

In anesthetizing an animal using the compounds and method of the presentinvention, the compound is usually administered by vaporizing thecompound in the presence of an innocuous gas vaporization medium suchas, for example, helium, nitrogen, oxygen or various mixtures thereof.The compound may also be in combination with other anesthetics such asnitrous oxide; for example.

From the foregoing discussion, it is seen that the deuterated analoguesof methoxyflurane, MF-D₄ and MF-D₃, have substantially the samedesirable anesthetic effects as methoxyflurane, but are not as readilymetabolized to inorganic fluorides by an animal which has inhaled them.Accordingly, it would be expected that the possiblity of renaldysfunction resulting from the presence of inorganic fluorides in theserum and urine would be significantly reduced by using MF-D₄ or MF-D₃as an anesthetic.

DETAILED DESCRIPTION OF THE INVENTION

The following examples will serve to further clarify the presentinvention but are not to be construed as a limitation thereon.

EXAMPLE 1 Preparation of 2,2-dichloro-1,1-difluoro-1-methoxy-d₃-ethane-d (MF-D₄)

Dowex-21K® (Dow) resin in the Cl form was converted to the OH-- form bywashing the beads with 10% hydrated potassium hydroxide. The beads werewashed with methanol and dried on a rotary-vacuum stripper.

The dried resin (10 grams) was charged along with 20 grams (0.56 mole)of fully deuterated methanol (CD₃ OD) into a 250 ml round bottom flaskequipped with a magnetic stirrer and cooled in an ice bath.Approximately 80 grams (0.6 mole) of 1,1-difluoro-2,2-dichloroethylenewas slowly bubbled into the flask over a period of about three hours.After this time, the ice bath was removed and the flask sealed andallowed to warm to room temperature overnight.

The resin was filtered away from the solution, and bromine was addeddropwise to destroy any excess olefin. The solution was washed withwater and distilled through a vigreux column. A center cut boiling at50°-51° C./110 mm Hg was confirmed as containing about 95%2,2-dichloro-1,1-difluoro-1-methoxy-d₃ -ethane-d using NMR data ascompared to an external standard.

EXAMPLE 2 Preparation of 1,1-difluoro-2,2-dichloro-1-methoxy-d₃ -ethane(MF-D₃)

Dowex-21K® (Dow) resin prepared in the manner of Example 1 above wascharged into a 3-necked 250 ml round bottom flask fitted with a largeefficient condenser, thermometer, addition tube, and magnetic stirrer.Deuterated methanol (CD₃ OD) (30 ml, 0.67 mole) was added to the flaskand cooled in an ice bath. To this mixture, 110.0 grams (0.827 mole) of1,1-difluoro-2,2-dichloroethylene (about a 20% excess) was added. Thereaction vessel was capped, allowed to warm slowly to room temperature,and maintained at about 22° C. for 18 hours.

The reaction mixture was separated from the resin beads by vacuumdistillation at 10 mm Hg. The liquid obtained was mixed with 50 ml of10% potassium hydroxide and refluxed for 0.5 hours and then separated byfractional distillation at atmospheric pressure. A proton NMR spectrumof a sample of collected material indicated the proton on the O-CF₂ CCl₂D group had been exchanged by hydrogen to about 80%. The1,1-difluoro-2,2-dichloro-1-methoxy-d₃ -ethane was brominated to ayellow color (about 3 drops of Br₂) and distilled under reducedpressure. The boiling point of the product was found to be 50° C. at 100mm Hg.

EXAMPLE 3

Metabolism studies for inorganic fluoride following use ofmethoxyflurane deuterated analogues described hereinbefore were carriedout as follows. Methoxyflurane, as a control, and MF-D₃, MF-D₄ and MF-Ddeuterated analogues thereof were vaporized by metering the liquidcompounds at a controlled rate into a temperature regulated vaporizationflask held at 150° C. The vapor was swept into the air inlet of a 30liter glass exposure chamber at a rate of 6 liters/minute. Theconcentration of the anesthetic in the exposure chamber was monitored bygas-liquid chromatography using direct gas sampling loops.

Groups of 6 male Fischer 344 rats (6 months of age, 200-300 grams) wereexposed to room air and 0.5% volume/_(volume) of methoxyflurane and eachof the three deuterated analogues described above for a period of 2hours. After the exposure, room air was passed through the chamber for30 minutes before the animals were removed. All animals were maintainedin individual metabolism cages for 48 hours after exposure. Urine wascollected during each of two 24 hour intervals after exposure. Urinaryvolume for each animal was recorded and the urine samples were assayedfor inorganic fluoride using an Orion fluoride electrode.

A comparison of the amount of total inorganic fluoride in the urine ofthe test animals receiving the deuterated compounds as compared tocontrols receiving methoxyflurane is shown in Table I below.

                  TABLE I                                                         ______________________________________                                        Compound    % Change in Fluoride*                                             ______________________________________                                         MF-D       +43%                                                               MF-D.sub.3 -34%                                                               MF-D.sub.4 -29%                                                              ______________________________________                                         *as compared to control animals treated with methoxyflurane.             

The data indicate, on the basis of residual inorganic fluoride in theurine following anesthesia that the compound2,2-dichloro-1,1-difluoro-1-methoxy-d₃ -ethane (MF-D₃) is the anestheticof choice with the compound 2,2-dichloro-1,1-difluoro-1-methoxy-d₃-ethane-d (MF-D₄) also showing a desirable significant reduction in theamount of inorganic fluoride in the urine.

In this study, no difference in the anesthetic properties ofmethoxyflurane and its deuterated analogues was noted.

It is understood that various modifications may be made in the exactmode of carrying out the present invention without departing from thespirit and scope thereof. While the foregoing description has beendirected to rats, it will be realized by one skilled in the art that thecompounds 2,2-dichloro-1,1-difluoro-1-methoxy-d₃ -ethane and2,2-dichloro-1,1-difluoro-1-methoxy-d₃ -ethane-d may be used with equalfacility on other inhalation anesthetic susceptible animals.

As noted above, the methoxyflurane analogues MF-D₄ and MF-D₃ are not asreadily metabolized to inorganic fluoride as measured in the urine of amammal which has inhaled the compounds when compared to mammals whichhave inhaled methoxyflurane or the analogue MF-D. While the inventordoes not wish to limit his invention to any particular mechanism, thefollowing proposed model is given to clarify the invention in light ofthe present art. This scheme is not to be taken as a limitation on thescope of the present invention.

Two major metabolites, methoxydifluoroacetic acid and dichloroaceticacid, have been found to result from the enzymatic degradation ofmethoxyflurane. On the basis of this knowledge, the metabolicdegradation of methoxyflurane could have two major pathways leading tothe elimination of the above metabolites. The proposed scheme is asfollows: ##STR4##

Assuming the proposed scheme is correct, then anesthesia with MF-D woulddecrease metabolism along Path I and force metabolism along Path IIwhich would result in higher inorganic fluoride in the serum. This iswhat has been observed. Anesthesia using the fully deuterated MF-D₄ orthe analogue MF-D₃ would either decrease the metabolism of the compoundsgenerally so that the compound is eliminated unchanged or alternativelyincrease the metabolism along Path I leading to less toxic metabolitesand lower levels of inorganic fluoride.

I claim:
 1. The compound 2,2-dichloro-1,1-difluoro-1-methoxy-d₃-ethane-d.
 2. The compound 2,2-dichloro-1,1-difluoro-1-methoxy-d₃-ethane.